Gingrich had a friendly telephone conversation Wednesday with Romney and had started planning an event where he would throw his support behind the likely nominee, Gingrich spokesman R.C Hammond said. The pair agreed to work together to unite conservatives against President Barack Obama.

"It's clear Romney is the nominee and the focus should be on defeating Obama. We should not focus on defeating ourselves," Gingrich told disappointed supporters in Kings Mountain, N.C., the morning after Romney tightened his grip on the nomination by sweeping primary contests in five states.

Gingrich also telephoned Republican National Committee Chairman Reince Priebus and supporters, such as Texas Gov. Rick Perry, in states with upcoming primaries to inform them of the decision he had been hinting at for days.

Gingrich had been under pressure for some time to leave the race and clear a path for Romney.

"You have to at some point be honest about what's happening in the real world as opposed to what you would like to have happened," he told supporters at a suburban Charlotte, N.C., restaurant.

Gingrich declined to comment when asked about his plans multiple times during the Kings Mountain stop.

"There are times when the mountain gets bigger than your ability to climb it," he said.

The White House acknowledged that the contest had come down to Obama and Romney.

"There seems to be a general acknowledgment that the process has moved to that stage," press secretary Jay Carney told reporters traveling with the president.

As the White House ratchets up its focus on Romney, Gingrich will shift to helping Republican candidates across the country, paying off more than $4.3 million in campaign debt and rebuilding his reputation among conservatives.

Gingrich's campaign tested conventional wisdom from the beginning. Could the 68-year-old grandfather - a politically divisive figure shamed by an ethics probe and subsequent reprimand, pushed out of congressional leadership and saddled with marital scandal - find acceptance among cultural conservatives?

His campaign was full of contradictions. He pointed to his 20 years as a congressman from Georgia, including four as House speaker, and claimed a political kinship with President Ronald Reagan. Yet he also contended to be an outsider and anti-establishment candidate.

While arguing for a less-intrusive federal government and dramatically lower spending and taxes, he promoted programs and initiatives with murky price tags, including establishing a colony on the moon and allowing younger workers to have private retirement accounts backed by the government.

He sought support from cultural conservatives even though he had married three times and committed adultery.

Gingrich's campaign lacked money and organization as it got under way, a problem that contributed to his failure to get on the ballot in Virginia, where he lives. Observers questioned whether the famously bombastic ex-congressman could maintain the message discipline needed for a successful national campaign. Within days of his formal announcement on May 11, 2011, he provided ample reason to think he could not.

First, Gingrich criticized a conservative-backed plan for revamping Medicare as promoting "radical change" and "right-wing social engineering," drawing an intense backlash from Republicans. He then grudgingly acknowledged that he and his wife, Callista, had a $500,000 credit account at the jeweler Tiffany and Co. but proclaimed "we are very frugal." Within weeks, the couple embarked on a cruise in Greece, which led to a mass exodus of campaign staff amid grumbling that he wasn't serious enough about his campaign.

Observers counted Gingrich out and considered him the victim of a self-inflicted implosion. At times during the summer and fall his campaign appeared to be more about selling his books and DVDs than running for the White House. His confrontational responses during debates - he often criticized the news media as well as Obama - played to conservatives who were eager to see Gingrich on stage with Obama.

Gingrich placed a distant fourth in the leadoff Iowa caucuses and repeated that showing in New Hampshire. Other candidates - Tim Pawlenty, Herman Cain, Michele Bachmann, Rick Perry, Jon Huntsman - dropped out before South Carolina voters went to the polls in January. Gingrich stayed in with the hope of becoming the leading alternative to Mitt Romney.

Gingrich found the reversal his campaign needed in South Carolina, site of the South's first primary. Defeating Romney there energized his campaign, but it invigorated Romney, too. He quickly turned his unmatched financial resources toward a negative TV campaign against Gingrich in Florida.

Romney won Florida's primary and Gingrich declared he would stay in the race through to the national convention in Tampa, Fla., in August - a declaration he continued to make as he racked up loss after loss in Nevada, Colorado, Minnesota, Maine, Arizona and Michigan. He wasn't on the ballot for Missouri's nonbinding primary.

Gingrich's strategy was to focus on Southern states, particularly Georgia. Gingrich did win Georgia, but it was one of only two victories he would achieve.

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